Hopefully works around travis failure until a proper fix.
To be precise, the macos build now fails with:
0pyenv: python3.6: command not found
1
2The `python3.6' command exists in these Python versions:
3
4 3.6
5
6 3.6.1
7
8make: *** [dist/Bitcoin-Qt.app/Contents/MacOS/Bitcoin-Qt] Error 127